The present invention relates to methods of detecting chemical compounds in liquids, and more particularly, to drug screening methods which are rapid, highly specific, and which can be performed on extremely small body fluid samples non-invasively or invasively obtained.
Prompt and accurate toxicologic laboratory information is important in the treatment of patients admitted to the hospital with drug overdose. Because the type of drugs involved in drug abuse are not often known, it is necessary to have both screening methods and definitive methods for identification, both of which must be able to be performed rapidly so that proper treatment can be administered as quickly as possible. In addition, a positive or negative finding eliminates the need for further extensive diagnostic work up.
A number of factors must be considered in selecting the type of screening and analytical methods for routine drug screening, and for and processing large numbers of samples. These factors include sensitivity, specificity, reproducibility, speed, simplicity, and cost. In addition, both the phase of the sample (i.e., solid, liquid, gas), and the ease of obtaining that particular sample must also be taken under consideration when selecting a detection method. Procedures currently available for drug detection in various body samples include thin layer chromatography, gas chromatography, high pressure liquid chromatography, various immunoassays, and gas chromatography-mass spectrometry.
Thin layer chromatography (TLC) has traditionally been used as a broad spectrum screen for drugs of abuse. This procedure is relatively inexpensive and does not require sophisticated instrumentation. However, TLC is insensitive; it yields qualitative (i.e., positive or negative) results only. Typically the minimum amount of drug or metabolite required to yield a positive result is in the 1-2 mg/ml range. Therefore, negative results may imply that the sensitivity of the method is inadequate for detection of drugs in the sample. Thus, the TLC screen is only useful for emergency room purposes where quick determination of a toxic level is necessary, and would not be used to detect low level substance abuse. Another problem with TLC is the relative lack of specificity. Under ideal conditions, a specific drug will always migrate to the same spot on TLC plates; however, similar molecules and other drugs may travel in approximately the same zone. Furthermore, TLC is labor intensive and may result in highly variable results between laboratories or even for a particular given laboratory.
Gas chromatography (GC) is an analytical technique that separates molecules by means of glass or metal tubing coated or filled, respectively, with material of particular polarity. The sample is vaporized at the injection port and carried through the column to the detector by a steady flow of gas. Separation of the compounds, and hence their detection is dependent on their interaction with the column packing. This detection method is limited because only gases and liquids with fairly low boiling points (e.g., methanol and ethanol) can be analyzed; most drugs of abuse have higher boiling points, and therefore, cannot be detected.
Gas chromatography-mass spectroscopy (GC-MS) is a method commonly used for confirmation of a positive analysis obtained by another screening method. GC-MS analyzes a liquid or gaseous substance according to its fragmentation pattern; the exact mass of the fragments is compared to a computer library resulting in a "fingerprinting" of molecules. The procedure is very costly and requires a high degree of technical expertise for its operation.
High pressure liquid chromatography (HPLC) is similar to GC with the exception that liquids rather than gas are used to propel substances through the columns. Unfortunately, this procedure is also time consuming and requires a high degree of technical expertise for its operation.
Immunoassays operate on the principle of antigen-antibody interactions, and may include the use of enzyme, fluorescent, or radioactive labels. Liquid samples can be easily analyzed. One widely used immunoassay for detecting toxic substances is the enzyme multiplied immunoassay (EMIT) system which works on the basis of an inhibition of an enzyme substrate reaction proportional to the amount of drug present in the sample (usually urine). Another useful analytical method is the fluorescence polarization immunoassay (FPIA) which has been applied to drug abuse testing in urine. (See, e.g., Jolley et al. (1981) Clin. Chem. 27:1575-1579; and Caplan et al. (1987) Clin. Chem. 33:1200-1202).
The reliability of any immunoassay depends on the specificity or sensitivity of the antibodies. Thus, the presence of compounds which cross-react with the antibody can result in a false negative or false positive result. In addition, a drug assay that is reported as negative may actually contain small amounts of drug diluted in a large volume of liquid sample. Other factors may also alter this sensitivity such as the pH and age of the sample, and the presence of contaminating substances (e.g., vinegar, soap) therein. Another disadvantage of immunoassay is the lack of specificity. Antibodies to drug antigens are notoriously non-specific and commonly used over the counter medications such as phenylisopropyl-amines, ephedrine, and phenylpropalamine react with antibodies in clinically obtained concentrations.
Ion mobility spectrometry is an analytical method that has been used to analyze gaseous samples, for example, in ambient atmospheres (Canadian Patent No. 1201646) and drug particles in contact with the skin surface (Nanji et al. (1987) Clin. Toxicol. 25:501-515). This technique distinguishes ionic species on the basis of the difference in the drift velocity of ions through gas under an applied electric field.
There is however, no known method of screening liquid samples of body fluids for a wide range of toxic compounds which is rapid, sensitive, accurate, and easy to perform.
Accordingly, it is an object of the present invention to provide a rapid, yet sensitive and specific method of drug screening.
It is another object of the invention to provide a method of detecting a wide range of toxic drugs in a liquid sample of a body fluid.
Another object to provide a method of detecting a plurality of chemical compounds in a body fluid sample.
Yet another object is to provide a non-invasive method of detecting a chemical compound in a body fluid.
Still another object is to provide a rapid method of detecting a chemical compound in a small volume of a body fluid.